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Astro-GR@Benasque, Benasque, Spain

The study of galactic nuclei has advanced rapidly during the past few years. Observations carried out with space-borne telescopes, such as the Hubble Space Telescope, or from the ground, using adaptive optics, have allowed us to study the kinematics of stars and gas in regions reaching down to sub-parsec scales for external galaxies, and to the milliparsec range for the Milky Way. An outstanding conclusion is that dark compact objects, very probably massive black holes (MBH), with masses ranging between a million and a thousand million solar masses, occupy the centres of most galaxies for which such observations can be made. We have discovered that there exists a deep link between the central MBH and its host galaxy. Claims of detection of “intermediate-mass” black holes (IMBHs, with masses between 100 and 10,000 solar masses) raise the possibility that these correlations extend to much smaller systems, but the strongest -if not totally conclusive- observational evidences for the existence of IMBHs are ultra-luminous X-ray sources. The origins of these IMBH are still shrouded in mystery, and many aspects of their interplay with the surrounding stellar cluster remain to be elucidated. A particularly important mode of interaction between stars and the MBH/IMBH is the disruption of stars by the strong central tidal field. This can trigger phases of bright accretion, possibly already observed in several galaxies as X-ray/UV flares. Secondly, collapsed stars such as neutron stars, white dwarves and stellar-mass black holes might be swallowed whole by the central MBH. This process is the result of a slow inspiral towards the event horizon because of the emission of gravitational waves (GWs). The detection of these small ripples of space and time constitute a unique and parallel way to probe the Universe, by reaching loci otherwise unaccessible to the photon.